140 Prof. J. G. MacGregor on Contact-Action 



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(4) Transference and Transformation of Energy. 



Prof. Lodge's argument to show that, on the assumption 

 of contact-action, " energy cannot be transferred without 

 being transformed/ 5 is as follows*: — " When a body possess- 

 ing potential energy does work, its ' range ' t necessarily 

 diminishes, while the motion of the body on which the work 

 is done increases. On the other hand, when a moving body 

 does work its motion diminishes, and the body which resists 

 the motion, since it yields over a certain distance, gains po- 

 tential energy."" It seems to me that these sentences would 

 be equally accurate if we were to subject them to " double 

 decomposition," after which process they would read thus : — 

 When a body possessing potential energy does work its range 

 necessarily diminishes, while the body on which work is done, 

 since it yields over a certain distance, gains potential energy. 

 On the other hand, when a moving body does work its motion 

 diminishes, while the motion of the body on which the work 

 is done increases. — A statement which would in general be 

 more complete than either, would be obtained by combining 

 the two. For if one body exert on another a certain force 

 through a certain distance the same work is done on it, 

 whether the former body lose kinetic or potential energy in 

 doing the work ; while the effect produced in the latter body 

 will in general be a change both in its motion and its state 

 of strain, i. e. both in its kinetic and its potential energy. 

 Thus, whether the former lose kinetic or potential energy, or 

 both, the latter will, in general, gain both, or transference of 

 energy will, in general, involve partial but not complete 

 transformation. 



Prof. Lodge cites the air-gun as an instance of the trans- 

 formation of potential into kinetic energy during transference. 

 If we extract the bullet and plug up the muzzle it will serve 

 equally well as an instance of the transference of potential 

 energy without transformation. For if we now pull the 

 trigger, the compressed air will do work on the air in the 

 barrel. The " range " of the former will diminish, that of 

 the latter will increase. As an instance of the transformation 

 of kinetic into potential energy during transference, he selects 

 the case of a bullet fired against a spring and caught by it. 

 But if the spring have inertia, the energy acquired by it 

 through the work done by the bullet must be partly kinetic, 

 and in such a case, therefore, the transformation is only 

 partial. Such instances, however, must be defective as illus- 



* Phil. Mag. [5] vol. xix. (1885) p. 486. 



f That is " the distance through which it can exert force." 



